The invention relates to a method and an apparatus for reconditioning worn-out electrical spot welding electrodes by means of a reshaping tool.
As a consequence of the pressure exerted on spot welding electrodes in their operation, and of the high temperatures to which they are exposed, the working surface of the electrodes is gradually deformed, especially in the sense of an enlargement of cross section at the usually conical, cylindrical, rounded or hemispherical tips of the electrodes. This is true even when, instead of fixedly mounted welding electrodes, replaceable, cap-like electrodes are used, which are replaceably fastened to mating electrode holders of the welding apparatus. Such surface deformations result in poor welds and hence in an impairment of product quality, and also in spattering during the spot welding operation. As a result, such electrodes have to be removed and replaced with new electrodes.
Until now, spot welding electrodes, especially cap-like electrodes of this kind, have been considered a disposable product. After a certain number of, for example, 2,000 to 15,000 welds, they are replaced and discarded as scrap metal. The cost of this practice is appreciable.
To reduce the consumption of new spot welding electrodes it is already known to recondition them, without removing them from the welding machine, by means of an annular shaping tool which is manually rotated one or more times about the electrode as center (French Patent No. 936,105). In this procedure, the manual rotation of the shaping tool is extremely difficult on account of the great amount of friction it entails, and on the other hand it takes a great number of rotations of the shaping tool to regain to any degree the desired shape of the reconditioned welding electrode. Moreover, the use of this known process is not easily possible on automatic welding assembly lines. Lastly, the quality of the spot welding electrodes reconditioned in this manner is very poor.
In the case of roller seam welding it is already known to remove the burrs that form on continuously revolving electrode rollers, or to prevent their formation, by means of rollers which constantly act on the welding surfaces (British Patent No. 282,960). This process, however, cannot be applied to the spot welding art.
It is furthermore known to recondition worn spot welding electrodes by machining operations such as turning, filing or milling, whereby the flattened and broadened working surfaces are restored to their initial state by cutting operations. Reconditioning processes of this kind, however, have not yet produced useful results. Furthermore, they are not feasible from the economic point of view.
It is furthermore known ("Schweisstechnik" 1968, vol. 1, pp. 17-20) that, in order to determine the useful life of spot welding electrodes, allowance must be made for the work-hardening of the contact surfaces that occurs during the first 1000 spot welds, and that the useful life of electrodes made from copper-chromium alloys can be substantially improved if the electrodes are made, not by machining, but by cold plastic deformation. Lastly, it is known ("DVS-Berichte Band 70" 1981, pp. 85-100) to produce electrode caps by cold plastic deformation. Processes such as these, however, have never previously been used for the reconditioning of worn spot welding electrodes.
It is the object of the invention to further develop the method and the apparatus of the kind referred to above to such a degree that the reconditioning of spot welding electrodes will be more economical and the reconditioned electrodes will be at least comparable in quality to new spot welding electrodes.
The method of the invention is characterized by pressing the worn spot welding electrode by means of a punch into a cavity formed in a die whose shape corresponds to the shape desired in the reconditioned spot welding electrode, doing so in such a manner that its working surface assumes the shape provided by the cavity, and such that the diffusion layers and/or alloy layers built up by previous use on and/or directly beneath the active working surface of the worn electrode will be preserved and will again be present on and/or directly beneath the active working surface of the reconditioned electrode.